Pat steak dry with paper towels. Combine Cajun seasoning, smoked paprika, garlic powder, salt, and black pepper in a small bowl. Rub spice mixture all over steak, coating both sides evenly. Let rest at room temperature for 10 minutes.

Bring a large pot of water to a rolling boil. Add 1 tablespoon salt and pasta. Cook according to package directions, 10 to 12 minutes, until al dente. Before draining, reserve ½ cup pasta cooking water. Drain pasta and set aside.

Heat olive oil in a large skillet over medium-high heat until shimmering. Add seasoned steak and cook undisturbed for 3 to 4 minutes per side for medium-rare, adding 1 to 2 minutes per side for more well-done. Transfer steak to a cutting board, tent with foil, and let rest for 5 minutes. After resting, slice thinly against the grain.

In the same skillet, melt butter over medium heat. Add diced onion and sliced bell peppers. Cook for about 5 minutes, stirring occasionally, until soft and slightly caramelized. Add minced garlic and cook 1 minute more until fragrant.

Pour in half-and-half and chicken broth, scraping up any browned bits from the bottom of the pan. Stir in Parmesan cheese, Cajun seasoning, and red pepper flakes if using. Reduce heat to low and simmer gently for 3 to 5 minutes until sauce thickens slightly.

Add drained pasta to the skillet and toss to coat with sauce. If sauce is too thick, add reserved pasta water a splash at a time until desired consistency is reached. Fold in sliced steak and any accumulated juices. Season with additional salt and pepper to taste. Cook together for 2 to 3 minutes until heated through.

Transfer to plates or a serving bowl. Top with fresh parsley, extra Parmesan cheese, and a squeeze of lemon juice if desired. Serve hot.